1. Field of the Invention
The present invention relates to the field of electric power generation and more particularly to cooling power plant generators with hydrogen using a closed loop control system.
2. Description of the Prior Art
Power plants take energy from coal, gas, nuclear or other fuels or from hydrostatic water to produce electricity for the electrical grid using electrical generators. These generators are large rotating electrical machines, and as such generate considerable heat do primarily to resistive electrical power losses as well as friction. Each of these generators must be cooled to remove this excess heat and hence keep the various parts of generator within a specified operating temperatures. A very common way of cooling the generators of a modern power generation plant is with hydrogen gas. Hydrogen gas is used because it is a very efficient coolant. The hydrogen supplied to cool a generator must have acceptable purity and water content (dew point). Typical pressure are maintained around 30 to 75 psig depending on age and size of the generator. As the generator operates, there is a continuous H2 gas leakage/usage resulting in pressure drop of the H2 pressure in the generator casing, A degradation in purity and dew point often occur over time as impurities and moisture are trapped in the system until a venting event, which generally is a manual release of casing gas. Generators have setpoints for maximum H2 pressure and alarms for low H2 pressure and low H2 purity, and possibly high dewpoint and high H2 gas outlet Temperature.
Since there is a small amount of continuous hydrogen leakage, when the hydrogen pressure begins to approach the lower allowable limit or alarm point, or on a regular time schedule (e.g. every day at 3:00 p.m., or broadly, such as the midnight shift), plant operators manually open a valve and repressurize the generator to its setpoint. If the purity is low or dew point is high, generator gas is vented and replaced with high quality hydrogen up to the pressure limit. This procedure is repeated until the desired purity or dew point is reached. Many systems may add a dryer specifically to maintain dewpoint as a single parameter.
Recently, equipment has been introduced that generates hydrogen on-site from water and other equipment that maintains the setpoints automatically. An example of this is HOGEN (Registered Trademark), and StableFlow (Registered Trademark) systems manufactured by Proton Energy Systems. The HOGEN® system is fed demineralized water and electricity and produces hydrogen gas using PEM technology, while the StableFlow® system automatically maintains a consistent setpoint for hydrogen pressure, hydrogen purity and hydrogen dew point. The control system automatically vents while generally adding 99.99% or better hydrogen gas simultaneously. In the United States most H2 is usually supplied by storage tanks and or cylinders and filled by truck deliveries, but comes a from a wide variety of sources. The actual source does not matter as long as the valving can supply gas to the generator in an automatic fashion.
The StableFlow® system samples hydrogen for purity and dew point and pressure from a hydrogn line, (e.g. H2 sample line) from the generator. For example, under the manual method, in a typical installation with setpoints of 60 psig hydrogen pressure, 99% purity and 20 degree F. dew point, the pressure might run from 57-60 psig, the purity from 97% to 99% and the dew point from 0 to 40 degrees F. Using StableFlow®, the pressure can be maintained at 60 psig ±0.2 psi; the purity can be maintained at 99%±0.2%, and the dew point can be maintained at 20 degree F.±2 degree F. all with no human intervention. StableFlow is a completely independent of the H2 gas source and only needs an open H2 regulator set to the desired P setpoint. The controls may tuned to differing tolerance levels and feedback timing, plus different configurations and quality of control equipment will affect the actual tolerance, but in all cases they permit the setpoints to be actively controlled, either singularly or together. Generally equipment like StableFlow® and HOGEN® are connected to the plant's distributed control system (DCS) so that monitoring can be handled remotely.
It is known in the art that a generator can operate over a range of temperatures and hence a range of hydrogen pressures. FIG. 3 shows a graph of this. For example, a unit with a maximum pressure rating of 60 psig and a temperature margin of 10 degrees F. at the hydrogen outlet gas could run as low as 50 psig while keeping the temperatures within manufacturers specifications. Of course, higher operating temperatures result in aging of organic materials in the generator core. Also, stator losses increase roughly 0.6% per degree F. (0.02% of the generator efficiency). However, it is also well known that there are considerable losses in the generator due to what is called windage. This is the fluid friction or gas resistance between the rotating generator parts and the hydrogen gas. These losses can be significantly reduced by running at lower hydrogen pressure. In the above example, if the generator were operated at only 50 psig, the windage loss is reduced by about 16%. For a 600 MW generator, that is around 480 KW of free power. Further, at lower hydrogen pressure, leakage is less, and hydrogen induced cracking is reduced. Also the work of the fans and pumps for the hydrogen and cooling water is reduced.
It is known in the art of alternating current machines that power is the combination of real power (Watts) which performs work and reactive power (VARs) which supports the magnetic fields required in the rotating devices in order for them to function. In an AC generator, an electric potential or output voltage is produced in the windings of a stator when an electrically unconnected rotor is rotated mechanically in the stator in a magnetic field. The magnetic field is produced and maintained by other windings (field windings) in the stator. Current flowing through the field windings in a perfect system would be purely reactive since it is used only to create a magnetic field. In a real generator, there are resistive losses in the field windings. Reactive power measured in VARs (volts times amps reactive) can be thought of as being supplied or used in a power system just as can real power or Watts that do actual work. VARs are generally thought of as being supplied onto the grid just as Watts are. A generator plant generally has a certain reserve capability to supply VARs into the system. Step up transformer reactive losses, reactive line losses of transmission lines and other losses can be thought of as using up VARs while the generator and hence the plant supplies VARs. As loads increase (the demand for electricity increases), VAR losses go up dramatically in the grid. For example, a 200 mile, 500 KV transmission line at a flow of 1200 MW requires around 400 MVARs (roughly the VAR output of 2 plants). However, if flow increased on that same line to 1500 MW because of increasing loads, the VAR losses go up to 900 MVAR. While the line is only supplying 300 MW more actual power it is “eating” 500 extra MVARs. Increased current on the line also leads to higher resistive line losses. In heavy line flows, the VAR losses can be 10 times greater than the resistive Watt losses. This all leads to decreasing end voltage. This results in reducing static VARs from capacitors by the square of the voltage and increased VAR losses at loads by the square of the current.
The dynamic generator reserves are related to its capability and are generally a function of the generator's cooling system. Generally higher hydrogen pressure leads to higher reserves. However, there are constraints: hydrogen pressure cannot be quickly increased. It may take about 30 minutes for a plant to move to higher pressure depending on pressure differential demanded, the pressure in the supply line, supply line width, casing volume, and other control and physical parameters of the specific configuration and equipment. Running continuously at higher pressure means higher costs for hydrogen losses and maintenance. There are various software programs available for power plants such as GenVARR™ developed by Southern Company, Inc. and the U.S. Department of Energy display present VAR and other status to plant operators on a real-time basis. This type of software can also show operators or controllers the status of multiple plants on a grid including their current VAR output and their reserve VAR capability.
It would be very desirable to be able to control generator pressure and other parameters such as purity and dew point dynamically based on capacity needed rather than maintaining the H2 pressure at or near the maximum allowed and other parameters addressed on a ‘as needed’ basis. The object could be to lower hydrogen pressure thus reducing windage losses and maintenance costs. Pressure can be increased when increasing loads on the plant demand more reserve VAR capacity. The state of VAR demand and voltage could be fed to a closed loop controller from dynamic monitoring software like GenVARR™.